CN113130745B - VO 2 @SiO 2 Nanoparticle filled type electro-phase change composite material and preparation method thereof - Google Patents
VO 2 @SiO 2 Nanoparticle filled type electro-phase change composite material and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a VO 2 @SiO 2 Nanoparticle filled type electro-phase change composite material and a preparation method thereof,relates to the technical field of electro-induced phase change composite materials. The method comprises the following steps: VO is to be provided with 2 Adding (M) nano particles into a mixed solution of deionized water and absolute ethyl alcohol, dispersing, adding ammonia water, stirring uniformly, adding ethyl orthosilicate ethanol solution with the pH value of the solution being 8.7-11.8, reacting for 8-24h, filtering, cleaning, and vacuum drying to obtain SiO 2 Coated VO 2 Nanoparticles, 0nm < SiO 2 The coating thickness is less than or equal to 3nm; siO is made of 2 Coated VO 2 Mixing the nano particles with PVP water solution, coating on a substrate, and drying to obtain VO 2 @SiO 2 Nanoparticle filled electrically induced phase change composites. The preparation method has simple steps and good effect; siO produced 2 Coating VO 2 The phase change nonlinear coefficient of the composite material filled with nano particles is obviously improved, and the phase change nonlinear coefficient is improved through SiO 2 Coating treatment not only improves VO 2 Is improved in the oxidation resistance of VO 2 The research result is for promoting VO (volatile organic compound) 2 Has important significance in the commercialized application of the product.
Description
Technical Field
The invention relates to the technical field of electro-phase change composite materials, in particular to a VO 2 @SiO 2 Nanoparticle filled type electro-phase change composite material and a preparation method thereof.
Background
In 1949, n.f. motto predicted the metal-oxide insulation-metal phase transition (MIT, metal-insulator transition) by band theory, and in 1959, morin first discovered single crystal vanadium dioxide (VO in bell laboratories 2 ) Phase transition phenomenon, VO therefrom 2 Phase transition characteristics are an important issue of interest in the field of condensed physics. As a typical strongly correlated system material, it undergoes a unique reversible metal-insulator phase transition (MIT) under the action of temperature, light and voltage. With the phase change, it shows drastic changes in optical, thermal and magnetic properties. At present, techniques such as magnetron sputtering, molecular beam epitaxy, sol-gel method and pulse laser deposition can be used for directly forming VO on a specific substrate 2 The requirements on equipment and process conditions are high, and the nano-structure VO is prepared by a hydrothermal method 2 Has the advantages of high purity, adjustable shape, flexible molding, and the like, and is a large-scale application of VO 2 Is a desirable choice of (c). But na (na)The vanadium dioxide powder has high thermodynamic instability due to small particles and large specific surface area and surface free energy. Is easily converted into V during long-term storage or in air at a temperature higher than 300 DEG C 2 O 5 And V is 2 O 5 Can be dissolved in an acidic environment to form VO 2+ ，V 3+ orV 5+ Ionic compound for greatly reducing VO 2 The service life of the powder is prolonged. On the other hand, vanadium dioxide is a typical inorganic metal oxide, and the lack of hydroxyl groups fused with an inorganic aqueous solution and carboxyl groups bonded with an organic substance on the particle surface makes the dispersion poor and the stability low, particularly in a hydrogel matrix. Uneven dispersion can further exacerbate the material's performance degradation, which is a significant limitation in application. Therefore, the surface modification treatment of the nano vanadium dioxide powder is necessary. At present, vanadium dioxide modification technology can be classified into metal ion modification, inorganic surface modification, organic surface modification and coating modification.
While many documents have demonstrated VO after surface coating treatment 2 Can obviously improve oxidation resistance and light transmittance, and is modified to treat nano VO 2 Efficient route of particles. Surface coating is to coat one or more layers of antioxidant substances such as titanium dioxide, silicon dioxide, zinc oxide and the like on the surface of the nano particles. The contact of vanadium element and oxygen ions in the space is blocked, the oxidation speed is greatly slowed down, and the service life is prolonged. The coating layer can provide more hydroxyl bonds for the particles, improve the dispersibility in aqueous solution, improve the effect of the silane coupling agent and improve the compatibility with other organic matters. In 2013, ping Jin team of China academy of sciences was improving VO 2 According to the solar energy regulation efficiency of (2), VO is synthesized based on a microemulsion method 2 (M)@SiO 2 And (3) nanoparticles. VO was prepared by the Stocker method based on Min Wang et al, shanghai university 2 @SiO 2 Core-shell nanoparticles, siO 2 The layer is smooth and uniform, the thickness is about 5nm, and the oxidation resistance temperature is improved by 25 ℃. University of eastern chemical industry, yanfang Gao et al, based on the modified Stocker method, uses PVP pretreatment to improve SiO 2 Is of shell-forming quality of (C)，SiO 2 The thickness of (2) is adjustable between 4 and 25 nm.
It can be seen that the oxidation resistance and dispersibility of the vanadium dioxide treated by the surface coating technology are obviously improved, the light transmittance is improved, but the coating treatment is carried out on VO 2 The influence of the electro-phase change properties of (c) is still unknown. In 2000, stefanovich et al reported for the first time that electric field or electron injection triggered VO 2 An insulation-metal phase transition phenomenon occurs. Electric field induced VO compared to thermal and optical excitation 2 The insulation-metal phase transition (E-MIT) has the advantages of high reaction speed, low loading cost, convenient integration, miniaturization, long service life and the like, and has wide application prospect in the aspects of reconfigurable antenna technology, terahertz technology, millimeter wave phase modulator, memory and neuron computer technology, rapid switching devices and the like. To improve VO 2 The oxidation resistance and dispersibility of the coating material, and the practical application of the coating material are required to be specially researched on the electro-phase change performance of the coating modified vanadium dioxide.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a VO 2 @SiO 2 The nanoparticle filled type electro-phase change composite material and the preparation method thereof have simple steps and good effect; siO produced 2 Coating VO 2 The phase-change nonlinear coefficient of the composite material filled with the nano particles is obviously improved, which indicates that the composite material is prepared by SiO 2 Coating treatment can not only improve VO 2 Can improve the oxidation resistance of VO at the same time 2 The research result is for promoting VO (volatile organic compound) 2 Has important significance in the commercialized application of the product.
In order to solve the technical problems, the invention adopts the following technical scheme: VO (VO) 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material comprises the following steps:
(1) VO is to be provided with 2 Adding (M) nanometer particles into a mixed solution of deionized water and absolute ethyl alcohol, dispersing, adding ammonia water, stirring uniformly, adding ethyl orthosilicate ethanol solution under stirring, continuing to react for 8-24h, and suction filteringCleaning, vacuum drying to obtain SiO 2 Coated VO 2 Nanoparticles, 0nm < SiO 2 The coating thickness is less than or equal to 3nm;
(2) Firstly, preparing polyvinylpyrrolidone PVP aqueous solution, and then adding SiO 2 Coated VO 2 Mixing the nano particles with PVP water solution, coating on a substrate, and drying to obtain VO 2 @SiO 2 Nanoparticle filled electrically induced phase change composites.
Preferably, in the mixed solution of deionized water and absolute ethyl alcohol used in the step (1), the volume ratio of the deionized water to the absolute ethyl alcohol is 1:3-1:5.
Further preferably, in the mixed solution of deionized water and absolute ethyl alcohol used in the step (1), the volume ratio of the deionized water to the absolute ethyl alcohol is 1:3.
Preferably, the concentration of the aqueous ammonia used in step (1) is 25wt% to 28wt%.
It is further preferred that the concentration of ammonia used in step (1) is 28wt%.
Preferably, the ethyl orthosilicate ethanol solution used in the step (1) is a 1% ethyl orthosilicate ethanol solution by mass fraction.
Preferably, step (1) is continued for 12h.
Preferably, VO 2 The mass ratio of the (M) nano particles to the tetraethoxysilane is 1:0.02-1:0.3.
Preferably, siO 2 Coated VO 2 SiO is less than or equal to 2nm in the nano particles 2 The coating thickness is less than or equal to 3nm.
Preferably, in the step (2), the mass concentration of the PVP aqueous solution is 2% -10%.
Preferably, in step (2), siO 2 Coated VO 2 The mass ratio of the nano particles to the PVP aqueous solution is 1:1-1:10.
Preferably, in the step (2), the substrate is a PCB board or glass. PCB is known as PrintedCircuit Board.
The VO described above 2 @SiO 2 The nanoparticle filled type electro-phase change composite material is prepared by the method for preparing the electro-phase change composite material.
The beneficial effects produced by adopting the technical proposalThe fruit is as follows: VO (VO) 2 As typical electron strongly-associated metal oxide, the electro-induced phase change characteristic of the metal oxide has great application potential in the fields of over-pulse protection, novel memory devices, field effect switches and the like. However, VO 2 The chemical instability of (c) and the like have limited their use in these fields. The invention successfully prepares the nanometer VO with different coating thicknesses 2 @SiO 2 Core-shell structured powder. Experimental test shows that the coated VO 2 Powder oxidation temperature ratio untreated VO 2 The powder was 45℃higher. While the phase transition temperature of the coated powder is still 68 ℃. By passing the prepared VO 2 @SiO 2 Mixing the powder with PVP organic matter to prepare VO 2 @SiO 2 A filled composite material. Test shows that in SiO 2 When the coating thickness is less than 3nm, the composite material still has the performances of temperature induced phase change and electric induced phase change. Under the action of temperature change and high voltage, the resistance can be suddenly changed. The material resistance increases with increasing cladding thickness. When the thickness of the material is higher than 3nm, the resistance of the material at normal temperature increases suddenly, the temperature induced phase change is not obvious, and repeated electric induced phase change cannot occur. Experiments show that the phase change nonlinear coefficient of the composite material filled with the coating particles is obviously improved, especially when the coating thickness is more than 2 nm. Description by SiO 2 Coating treatment can not only improve VO 2 Can improve the oxidation resistance of VO at the same time 2 The optimal coating thickness is between 2nm and 3nm. Research results for promoting VO 2 Has important significance in the commercialized application of the product.
Drawings
The invention will be described in further detail with reference to the drawings and the detailed description;
FIG. 1a shows VO of sample # 1 (nTEOS=0μl) in example 1 of the present invention 2 SEM image of nanoparticles;
FIG. 1b shows VO of sample # 1 (nTEOS=0μl) in example 1 of the present invention 2 Nanoparticle electronic heat dissipation energy spectrum;
FIG. 1c shows VO for sample # 5 (nTEOS=150. Mu.l) in example 1 of the present invention 2 @SiO 2 SEM image of nanoparticles;
FIG. 1d is the present inventionSample # 5 (nTEOS=150 μl) VO in example 1 2 @SiO 2 Nanoparticle electronic heat dissipation energy spectrum;
FIG. 2 shows VO with different TEOS addition in example 1 of the present invention 2 @SiO 2 TEM image of nanoparticles; wherein (a) no TEOS was added to the comparative sample; (b) TEOS in an amount of 20. Mu.l; (c) TEOS in an amount of 50. Mu.l; (d) TEOS in an amount of 100. Mu.l; (e) TEOS in an amount of 150 μl; (f) TEOS in an amount of 300. Mu.l;
FIG. 3 is SiO in example 1 of the present invention 2 A graph of the relationship between the thickness of the cladding layer and the TEOS addition;
FIG. 4 is VO in example 1 of the present invention 2 @SiO 2 XRD pattern of nanoparticles;
FIG. 5 shows sample VO of example 1# 1, 3# 3 and 5# of the present invention 2 @SiO 2 XPS measurement spectrogram of the nanoparticles;
FIG. 6 shows sample VO for sample # 1, # 3 and # 5 in example 1 of the present invention 2 @SiO 2 A high resolution XPS spectrogram of the nanoparticle; wherein, (a) O1s; (b) V2p (c) Si2p;
FIG. 7a is a sample VO prepared in example 1 of the present invention 2 @SiO 2 DSC test profile at 40 ℃ to 600 ℃ for nanoparticles;
FIG. 7b is a sample VO prepared in example 1 of the present invention 2 @SiO 2 DSC test profile at 50 ℃ to 100 ℃ for nanoparticles;
FIG. 8a is a sample VO prepared in example 1 of the present invention 2 @SiO 2 Thermogravimetric TG profile of nanoparticles;
FIG. 8b is a sample VO prepared in example 1 of the present invention 2 @SiO 2 A differential thermogravimetric DTG profile of the nanoparticle;
FIG. 9 is a sample VO prepared in example 1 of the present invention 2 @SiO 2 A plot of the relationship between the oxidation temperature of the nanoparticle and the thickness of the coating layer;
FIG. 10 is VO in example 1 of the present invention 2 @SiO 2 Temperature-induced phase transition curve graph of PVP composite film;
FIG. 11 is VO in example 1 of the present invention 2 @SiO 2 PVP composite film phase change front-back resistanceA variation graph;
FIG. 12 is VO in example 1 of the invention 2 @SiO 2 6 samples of PVP composite film; (a) sample # 1; (b) sample # 2; (c) sample # 3; (d) sample # 4; (e) sample # 5; (f) sample # 6;
FIG. 13 is VO in example 1 of the invention 2 @SiO 2 The PVP composite film 2# sample is tested for a system voltage and current curve of an electric phase change curve at the 5 th time and a system voltage and voltage curve of two ends of the composite material;
FIG. 14a is VO in example 1 of the invention 2 @SiO 2 A phase change voltage and cladding thickness relation graph of PVP composite material;
FIG. 14b is VO in example 1 of the invention 2 @SiO 2 A curve graph of nonlinear coefficient and coating thickness of PVP composite material system;
FIG. 15 is VO in example 1 of the present invention 2 @SiO 2 PVP composite material conductive model diagram.
Detailed Description
Example 1
VO 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material comprises the following steps:
VO 2 (M) synthesis of nanoparticles:
firstly, 1 part of oxalic acid is dispersed in 10 parts of deionized water, then 0.5 part of vanadium pentoxide is added into the mixed solution, and the mixed solution is fully stirred and mixed to obtain a yellow solution.
The yellow solution was transferred to a 200ml polytetrafluoroethylene-lined stainless steel autoclave at a constant temperature of 180℃to 240℃for 4h to 48h. Naturally cooling to room temperature after finishing constant temperature, respectively washing with absolute ethanol and deionized water, suction filtering, and freeze drying for 12 hr to obtain blue powder VO 2 (B)。
VO to be obtained 2 (B) Vacuum annealing the sample in a tube furnace at 500-650 deg.c for 30-90min to obtain M phase VO 2 And (3) nanoparticles.
VO 2 @SiO 2 Synthesis of nanoparticles:
first, 1g is preparedPrepared VO 2 (M) nanoparticles were added to 400ml of a mixed solution of deionized water and absolute ethanol (volume ratio of water to absolute ethanol 1:3). Dispersing for 30min by ultrasonic, adding 2ml of 28wt% ammonia water, and stirring uniformly to obtain a solution with the pH value of 10.17. Then, a certain amount of 1wt% ethyl orthosilicate (TEOS) ethanol solution was dropped in 10min under high-speed stirring. The reaction was continued for 12h. Washing with deionized water and alcohol for three times, vacuum filtering to obtain blue sample, vacuum drying at-35deg.C under vacuum degree of 8Pa to obtain SiO 2 Coated VO 2 And (3) nanoparticles. SiO adjustment by addition of different amounts of TEOS 2 The thickness of the coating layer was determined to be 1-6# for samples with TEOS addition amounts of 0. Mu.l, 20. Mu.l, 50. Mu.l, 100. Mu.l, 150. Mu.l, and 300. Mu.l, respectively.
VO 2 @SiO 2 Synthesis of nanoparticle filled type electro-phase change composite material:
firstly, preparing PVP aqueous solution with the concentration of 5wt percent, and then adding SiO 2 Coated VO 2 Mixing the nanoparticle 1-6# sample with PVP solution at a mass ratio of 1:4, coating the mixed solution on a substrate, which can be a PCB, glass, etc., and drying to obtain VO 2 @SiO 2 Nanoparticle filled electrically induced phase change composite sample 1-6. All chemicals used in the experiment were analytical grade and no further purification was required.
Characterization of
Synthesis of VO by various methods 2 @SiO 2 The nanoparticles were characterized. XRD patterns were obtained on an X-ray polycrystalline diffractometer (XD 6, beijing ps general instruments limited). Compounds were identified by comparison with the powder diffraction standard (JCPDS). VO was performed at 5kv using a high resolution scanning electron microscope (SEM, gemini-SEM-300SEM instrument, germany) 2 The microstructure of the powder was studied. In addition, VO was analyzed by transmission electron microscopy (TEM, JEOL-JEM-2100TEM instrument, japan) at 200 kilovolts 2 Is a feature of (3). For accurate measurement of coating treatment on VO 2 And the influence of the coating atoms, the separation of the nanoparticles using X-ray photoelectron spectroscopy (XPS Thermo Fisher Scientific K-Alpha)And analyzing the substructure, the atomic valence state and the element content. The phase change and oxidation properties of the prepared powder were tested using a synchronous thermal analyzer (SDT Q600 Simultaneous DSC-TGA). In the experiment, the sample was placed in an open aluminum crucible with air as an oxidizing atmosphere.
Testing VO in incubator 2 @SiO 2 The resistivity of the nanoparticle filled type electrically-induced phase change composite material changes with temperature. And the temperature and the material resistance are recorded by using a material resistance tester. The dc phase change characteristics were tested using keyhley 2657A. To prevent the current from being excessively large, a resistor of 2kΩ or 8kΩ is selected to be connected in series in the test circuit according to the initial resistance of the material, and the maximum current is set to be 50mA or 20mA.
Results
FIGS. 1a and 1c show 1#VO without TEOS addition, respectively 2 Nanoparticles and VO after coating treatment with 150 μl TEOS 2 SEM image of nanoparticles. As can be seen from the figure, the VO after the coating treatment 2 @SiO 2 Nanoparticle and original VO 2 The appearance of the nano particles is basically the same, and the coating treatment does not change the appearance of the particles. And no new impurity particles are formed in the material, such as SiO 2 Particles, and the like. And FIGS. 1b and 1d are X-ray spectroscopy spectra of the sample. It can be seen that the two groups of samples consist essentially of V and O elements, with an element ratio of about 1:2, to VO 2 The vanadium-oxygen ratio in (a) is the same. The Si element in sample # 1 is very small and can be considered to be caused by equipment errors. When the TEOS addition amount was 150. Mu.l, a significant peak position of Si element appeared in the EDS spectrum. The Si content in the particles was 1.1%. This is because VO is after coating treatment 2 The nanoparticle surface forms Si or an oxide of Si.
For further observation of the prepared SiO 2 Coated VO 2 The nanoparticle was subjected to transmission electron microscopy on the sample as shown in fig. 2. In the case of (a) without the addition of TEOS, the particle surface was relatively smooth and no significant color differences occurred. And after a small amount of TEOS is added, VO 2 The surface of the particles is obviously coated. When n is TEOS At =20 μl (b), the coating thickness is about 1.1nm; when (when)n TEOS When =50μl, the coating thickness was between 1.4nm and 2.9nm, the coating thickness was not uniform, and the individual parts were not completely coated, as shown in the circle of fig. 1 c. The imperfection of the coating will favor oxygen ions and internal VO 2 Contact is detrimental to the oxidation resistance of the particles. And when the TEOS is added in an amount of more than 100. Mu.l, the particle surface may form a smooth and uniform coating. As can be seen from FIG. 3, the thickness of the coating layer increases with the increase of the addition amount, but the increase rate becomes slow because VO is produced in the solution process 2 @SiO 2 When nanoparticles are formed, siO 2 The coating layer depends on VO 2 Adsorbing hydroxyl on the surface, and performing VO after high-temperature treatment 2 The amount of surface hydroxyl groups is small, the thickness of the coating layer is increased, and the adsorption force is weakened, so that the VO which is not modified by the surfactant 2 SiO of (2) 2 The thickness of the coating layer is smaller.
FIG. 4 shows the original VO 2 Nanoparticles and SiO 2 XRD profile of the coated nanoparticle. The results showed that all diffraction peaks of the samples were equal to VO 2 (M) Standard card (JCPDS-PDF#81-2392). The four main diffraction peaks at 2θ= 27.795, 37.088, 42.268 and 55.450 are assigned to the (011), (200), (210) and (220) crystal planes, respectively. The coating treatment did not change VO 2 The crystal structure of the nanoparticle. SiO was not found in the figure 2 Can infer SiO formed in aqueous solution 2 The coating layer is amorphous.
VO 2 The surface coating of the nanoparticles can be further determined by XPS testing. XPS full spectra of samples # 1, # 3 and # 5 are shown in FIG. 5. XPS data herein is corrected for C1s binding energy 284.8 eV. VO in FIG. 5 2 After the nano-particles are coated, sharp Si2p and Si2s peaks appear on the binding energy of 103eV and 150 eV. Indicating that SiO was formed in the coated sample 2 A compound. The intensity ratio of V2p to O1s was reduced in XPS spectra of 3# and 5# compared to the uncoated sample of 1 #. This is because of the coating layer SiO 2 Reduce the VO of the nuclear structure 2 To verify the SiO 2 Exist in VO 2 A surface. VO (VO) 2 And VO (Voice over Internet protocol) 2 @SiO 2 The narrow spectra of O1s, V2p and Si2p of (C) are shown in FIG. 6. As can be seen from FIG. 6 (a), V-O bonds (. About. 529.88 eV) appear in all three groups of samples, illustrating the presence of vanadium-containing oxides in the samples. In FIG. 6 (a), a significant peak position was also observed at the binding energy of 531.8 eV. Analysis shows that this is sample VO 2 Surface-adsorbed oxygen such as free hydroxyl groups (-OH) and water (H) 2 O). This is because of pure VO 2 The oxygen in the air is easy to be absorbed by the air, so that the oxidation reaction is promoted. As can be seen from the analysis middle and upper graphs, the Si-O peak was differentiated from the O1s after coating, and the adsorbed oxygen peak at 531.8eV disappeared. Further description of the presence of SiO in the sample 2 And the coating reduces the ability of the sample to adsorb oxygen. At the same time, it can be found that SiO increases with the coating thickness 2 Is increased. This is because the photoelectron penetration thickness is small in XPS test, and the thickness of the cladding layer is large at a certain sampling thickness, and the SiO in XPS sampling is large 2 The higher the content. While in the narrow spectrum of FIG. b V p V appears simultaneously 4+ And V 5+ Electron diffraction peaks. This is because of VO 2 Oxidation reaction takes place in air to produce V 2 O 5 . While V appears in the coated sample 2 O 5 The possible reason is that the sample has been oxidized prior to the coating process. By semi-quantitative analysis of V in three samples 4+ And V 5+ Ratio (1 #:65.26:34.74,3# -74.77:25.23, 5# -75.15:24.85), it can be seen that V in the coated samples 2 O 5 The content is obviously reduced. Can be explained as SiO 2 The coating layer retards VO 2 Oxidation reaction of the surface. Comparison of sample V in FIG. 6 (b) 4+ And V 5+ Is a binding energy position of (a). After the coating treatment, the binding energy of V is reduced because V-O-Si bonds are formed on the V surface, whereas-O-Si groups are relative to V 4+ The electron-pushing property is achieved, so that electron cloud density around the V atomic nucleus is increased, shielding effect is enhanced, and electron binding energy is reduced. As is more evident from FIG. 6 (c), siO is present in the treated sample 2 . To sum up, the treated VO was analyzed 2 The surface of the nano particle is successfully coated with amorphous SiO 2 VO is reduced 2 Oxygen adsorption capacity is improvedOriginal VO 2 Is used for the oxidation resistance of the steel sheet.
To verify the coated sample VO 2 @SiO 2 The oxidation resistance of the nanoparticles TG-DSC test was performed on the prepared samples in the 600 ℃ range (as shown in fig. 7a, 7b, 8a and 8 b). As can be seen in fig. 7a, there are clearly two peaks in the DSC test curve (endothermic peaks below 50 degrees are caused by systematic errors). The low temperature peak-to-peak of all samples was concentrated at VO 2 (M) around 68℃as shown in FIG. 7 b. The coating treatment does not affect VO 2 Transition from low temperature M to high temperature R phase. And the high temperature exothermic peak is concentrated between 350 ℃ and 500 ℃. As can be seen from the mass change curve (TG) (fig. 8), the sample mass was substantially unchanged before the high temperature exotherm peak. At the high temperature exotherm peak, the sample mass increased by about 9%. According to VO 2 Oxidation equation, assuming VO 2 All oxidized to the highest valence compound V 2 O 5 ：
2*VO 2 +0.5*O 2 ＝V 2 O 5 (1)
It can be known that VO is known by calculation 2 The mass increase after complete oxidation was 9.6%. Is completely consistent with the mass increase at the high temperature exothermic peak in the TG curve. It can be noted that the high temperature exothermic peak in FIG. 7 is VO 2 Oxidation reaction in an air atmosphere. And as can be seen in FIGS. 7b and 8b, with SiO 2 The thickness of the coating increases, and the exothermic peak moves in the high temperature direction. And the relationship between the oxidation temperature and the coating thickness of FIG. 9 can be explained for SiO 2 Coating can obviously improve VO 2 And the thicker the coating, the stronger the oxidation resistance. But the thickness of the coating layer is more than 2nm 2 @VO 2 The oxidation resistance of the nanoparticle is no longer improved and the maximum oxidation temperature is about 470 ℃.
VO of the prepared sample 2 @SiO 2 Mixing the nano particles with PVP to prepare VO 2 @SiO 2 A base composite material. Fig. 10 shows the resistance change curve of the composite film during the temperature increase and decrease. The prepared sample can generate temperature-induced phase change behavior in the heating and cooling processes。SiO 2 The coating treatment did not change VO 2 Is a phase change characteristic of (a). Fig. 11 shows the trend of the resistance and the change rate of the composite material with the coating thickness h under the conditions of high temperature and low temperature. It can be seen that the resistance of the composite increases linearly with increasing coating thickness, while the rate of change of resistance before and after phase change remains substantially unchanged, about 210 times. When the coating thickness is more than 3nm, the material resistance is obviously increased, and the resistance change rate is obviously reduced.
To further analyze VO 2 @SiO 2 The resistance response characteristics of the base composite under the action of voltage were measured 5 times for each of the 6 samples. The test curves are shown in fig. 12. In order to ensure that the material is not damaged by current abrupt change in the test, a protection resistor is added in the test circuit. According to the phase change characteristics of the material, a 2k omega resistor is connected in series in a 1-4# sample test, and an 8k omega resistor is connected in series in a 5# sample test and a 6# sample test. It can be seen that all samples except sample # 6 underwent abrupt resistance changes, i.e., electrically induced phase changes, during the voltage rise. And the 6# sample has higher phase change voltage due to higher initial resistance, and is damaged due to high-voltage breakdown in the first test. In combination with the temperature-induced phase transition curve, it can be stated that when VO 2 SiO on the surface 2 When the coating layer is larger than 3nm, VO is caused due to the too large thickness of the insulating layer 2 The initial resistance and the phase change voltage of the nano particles are too high, and the phase change performance of the material is poor. As can be seen from an analysis of fig. 12, the phase change voltages of the first test are higher than the following test results. After the first test, the phase change performance of the material is relatively stable. In actual use, factory prefabrication activation can be performed, and then the product is mainly used in a stable state. Since the material was relatively stable after the first test and the 6# sample had no repeatable electro-induced phase change properties, the 5 th test data of the 1-5# samples were selected for analysis.
Fig. 13 shows a fifth test curve for sample # 2. Wherein the left curve is the V-I curve of the test system, and the right curve is the voltage V at two ends of the material during the test 0 With the system input voltage V i Is a relationship of (2). It can be seen that the voltage across the material suddenly decreases during a phase change. This is because of the electricity in the materialThe resistor is suddenly changed under the action of voltage, and the partial pressure of the material is reduced. The voltage across the material reaches a maximumAt this point the material starts to change phase, defining +.>Phase change voltage V of material MIT . When the voltage across the material is no longer decreasing +.>The phase change of the material is ended. Whereas beta=log (I) is defined in terms of nonlinear coefficients 2 /I 1 )/log(V 2 /V 1 ) The nonlinear coefficient of the material was found to be 95.58. The nonlinear coefficient represents the abrupt change performance of the material under the action of voltage.
Table 1 shows the phase change voltage and the nonlinear coefficient during the electro-induced phase change. In combination with the variation graphs of FIGS. 14a and 14b, it can be seen that SiO 2 The coating thickness can improve the phase change voltage of the material and increase the nonlinear coefficient of the material. And as can be seen from fig. 14b, when the cladding thickness is higher than 2nm, the electro-induced phase change nonlinear coefficient of the material is abrupt, and the phase change voltage of the material is higher than 100V. As can be seen from the above analysis, the coating thickness is between 2nm and 3nm, and the VO can be improved 2 Improves the phase change performance of the material while resisting oxidation.
Table 1 VO 2 @SiO 2 PVT electro-phase transition curve data
Discussion of the invention
To analyze coating treatment vs VO 2 The influence of the electro-induced phase change establishes VO 2 @SiO 2 Composite conductive pattern (as shown in fig. 15). VO in composite material 2 The mass fraction has exceeded the percolation threshold. Then in the composite material VO 2 The nanoparticles are in contact with each other. As can be seen from fig. 15, VO is present in the composite material 2 @SiO 2 The conductive filler overlaps the conductive channels formed. And a cross-sectional view of the conductive filler lap joint is shown in the left-hand inset of figure 15. VO formation at nanoparticle contact points 2 -SiO 2 -VO 2 A potential barrier. The barrier height is formed by coating material SiO 2 And (5) determining. The resistance of the conductive material can be divided into VO 2 A resistance R1 formed of nanoparticles and a contact resistance R2 formed of SiO 2. Then the total resistance
R＝n1*R1+n2*R2 (1)
For sample No. 1 which is not coated, the coating is made of SiO 2 The resistor R2 formed is absent. At this time, the total resistance of the material is VO 2 The resistance determines the phase change performance of the material only by VO 2 And (5) particle determination. For VO 2 After the nano particles are coated, a coating formed by 2 layers of SiO 2 SiO formed by coating layer 2 Contact resistance R2. According to quantum tunneling theory, carriers pass through barriers between interfaces in two main ways, high-energy electrons form transition current in a transition mode, low-energy electrons form tunneling current in a quantum tunneling mode, total current increases linearly under low voltage, but current changes suddenly under high voltage, and nonlinear response is formed. Although SiO 2 Normally an insulator, but the thickness of the coating is nano-scale, and the VO after coating treatment 2 @SiO 2 Tunneling current formed by electron tunneling can be formed between the particles, forming a contact resistance R2. But quantum tunneling can only occur between interfaces of very small thickness (typically in the order of nm). The total resistance of the 2-6# composite material is formed by VO 2 Resistor R1 and contact resistor R2. In the coating treatment samples, the same batch of VO was used 2 Nanoparticles, so in all samples, by VO 2 The resulting resistance R1 is the same. Resistor R2 along with coating layer SiO 2 The thickness increases and increases. According to equation 1, the composite resistance R increases with increasing thickness. Consistent with the experimental results in fig. 8 a.
When the external electric field reaches the composite material phase change voltage VMIT, VO 2 Nanoparticles phase under the drive of Mott mechanismThe resistor R1 changes suddenly. At the same time under the action of high pressure, siO 2 The number of tunneling electrons in the contact surface increases sharply, and the resistance R2 simultaneously becomes abrupt. VO at this time 2 Phase transition properties and contact area SiO of (C) 2 The abrupt change of resistance in the composite material determines the phase change performance of the composite material. VO increases with the coating thickness h 2 Phase change properties of (2) are unchanged, but SiO 2 The resistance mutation of the contact surface is improved, so that the phase change voltage and the nonlinear coefficient of the composite material are improved when the composite material changes phase. Consistent with the experimental results in fig. 14a and 14 b. But quantum tunneling of electrons only occurs with small thickness. After the thickness is increased, the quantum tunneling effect disappears. Therefore, when the thickness of the coating layer is larger than 3nm (such as a 6# sample), the initial resistance of the composite material is close to 1MΩ, and the electro-induced phase change property of the material disappears.
The invention provides a method for preparing a silicon oxide film by SiO 2 Coating to improve VO 2 Methods for particle oxidation resistance and electro-induced phase change properties. Firstly, based on the method of the invention, the addition amount of TEOS is adjusted to successfully prepare VO with coating layers of different thicknesses 2 @SiO 2 And (3) nanoparticles. Experiments find that SiO 2 The thickness of the coating increases with the amount of TEOS added, up to about 3nm. As can be seen from DSC test, the prepared VO 2 @SiO 2 The oxidation resistance of NPs is higher than that of untreated VO 2 NPs can be increased by 45 ℃. The experiment mixes the prepared sample with PVP to obtain the composite film with PVP as organism. Through the resistance of the test sample under the action of temperature rise and high pressure, the resistance mutation phenomenon of the composite film can be found when the thickness of the coating layer is smaller than 3nm. But has a higher phase change voltage than the untreated composite film. This is because of SiO 2 As an insulator, the barrier height between particles is significantly increased, and the initial resistance and the phase change voltage are significantly increased. The optimal coating thickness is between 2nm and 3nm for optimal electro-induced phase change performance. The research results can improve VO 2 The service life of the nano particles in the use process is prolonged, the electro-induced phase change performance is improved, and the VO is realized 2 The practical application of the nano particles has important significance.
Claims (8)
1.VO 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material is characterized by comprising the following steps of: the method comprises the following steps:
(1) VO is to be provided with 2 Adding (M) nano particles into a mixed solution of deionized water and absolute ethyl alcohol, dispersing, adding ammonia water, stirring uniformly, adding ethyl orthosilicate ethanol solution under stirring, continuing to react for 8-24h, filtering, cleaning, and vacuum drying to obtain SiO 2 Coated VO 2 Nanoparticles, siO 2nm or less 2 The coating thickness is less than or equal to 3nm; VO (VO) 2 (M) the mass ratio of the nano particles to the tetraethoxysilane is 1:0.02-1:0.3;
(2) Firstly, preparing polyvinylpyrrolidone PVP aqueous solution, and then adding SiO 2 Coated VO 2 Mixing the nano particles with PVP water solution, coating on a substrate, and drying to obtain VO 2 @SiO 2 Nanoparticle filled electrically induced phase change composites.
2. The VO of claim 1 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material is characterized by comprising the following steps of: in the mixed solution of the deionized water and the absolute ethyl alcohol used in the step (1), the volume ratio of the deionized water to the absolute ethyl alcohol is 1:3-1:5.
3. The VO of claim 1 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material is characterized by comprising the following steps of: the concentration of the ammonia water used in the step (1) is 25-28 wt%.
4. The VO of claim 1 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material is characterized by comprising the following steps of: the ethyl orthosilicate ethanol solution used in the step (1) is 1% ethyl orthosilicate ethanol solution by mass fraction.
5. The VO of claim 1 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material is characterized by comprising the following steps of: step (2)The mass concentration of PVP aqueous solution is 2% -10%.
6. The VO of claim 1 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material is characterized by comprising the following steps of: in step (2), siO 2 Coated VO 2 The mass ratio of the nano particles to the PVP aqueous solution is 1:1-1:10.
7. The VO of claim 1 2 @SiO 2 The preparation method of the nanoparticle filled type electro-phase change composite material is characterized by comprising the following steps of: in the step (2), the substrate is a PCB or glass.
8. The VO of any one of claims 1 to 7 2 @SiO 2 The nanoparticle filled type electro-phase change composite material is prepared by the method for preparing the electro-phase change composite material.
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